NMR STUDIES OF LIQUID Cs-I SOLUTIONS

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NMR STUDIES OF LIQUID Cs-I SOLUTIONS

S. Sotier, W. Warren, Jr

To cite this version:

S. Sotier, W. Warren, Jr. NMR STUDIES OF LIQUID Cs-I SOLUTIONS. Journal de Physique

Colloques, 1980, 41 (C8), pp.C8-40-C8-43. �10.1051/jphyscol:1980810�. �jpa-00220194�

JOURNAL DE PHYSIQUE ColZoque C8, suppldrnent au n o 8, Tome 41, aoiit 1980, page C8-40

N M R STUDIES OF LIQUID Cs-I SOLUTIONS

S. S o t i e r X and W.W. Warren, J r .

Be22 Laboratopies, I n c . , Murray H i Z Z , New Jersey 07974, USA

A b s t r a c t . - The metal-nonmetal t r a n s i t i o n i n l i q u i d C s - I s o l u t i o n s h a s been i n v e s t i g a t e d by and l Z 7 1 NMR over t h e range from pure C s t o approximately Cs12. With i n c r e a s i n g I c o n c e n t r a t i o n s i n Cs, we observe a s h a r p d e c r e a s e of t h e 1 3 3 ~ s Knight s h i f t u n t i l f o r s t o i c h i o m e t r i c C s I and I - r i c h s o l u t i o n s o n l y a small chemical s h i f t i s observed. "'I s h i f t s i n d i c a t e s u b s t a n t i a l ene- t r a t i o n of conduction e l e c t r o n charge t o t h e n u c l e i of I- i o n s i n Cs-rich s o l u t i o n s . The 13'Cs s p i n - l a t t i c e r e l a x a t i o n r a t e s r e v e a l a g r a d u a l breakdown of n e a r l y - f r e e - e l e c t r o n c o n d i t i o n s and p r o g r e s s i v e l y s t r o n g e r l o c a l i z a t i o n a s I i s added t o C s . Below a few p e r c e n t e x c e s s Cs i n C s I

t h e d a t a i n d i c a t e f o r m a t i o n of l o c a l i z e d s t a t e s whose s t r u c t u r e i s analogous t o F-centers.

I. I?CROi:UCTION

The metal-nonmetal (PI-IiTI) t r a n s i t i o n i n l i q u i d m e t a l - s a l t s o l u t i o n s d i f f e r s from those i n o t h e r c l a s s e s of l i q u i d systems(expanded m e t a l s , chalcogens, metal-ammonia) because of t h e v e r y s t r o n f coulomb i n t e r a c t i o n s of conduction e l e c - t r o n s w i t h t h e i o n i c medium /I-3,'- These sol-&%one, .especiaU.y She oonceptua1;ly simple a l k a l i m e t a l - a l k a l i h a l i d e solu- t i o n s , a r e a l s o of i n t e r e s t a s paradigms f o r t h e so-called " i o n i c a l l o y s " such a s Cs-Au which t h e y resemble i n many r e - spec's /2-3/. Of t h e a l k a l i metal-halide s o l u t i o n s , only C s and i t s h a l i d e s a r e f u l l y m i s c i b l e over t h e f u l l range of composition /1,4/. For t h i s reason and because of t h e f a v o r a b l e NMl7 p r o p e r t i e s of 1 3 3 ~ s m d 1 2 7 ~ , we have chosen C s - C s I - I f o r t h i s f i r s t N M R i n v e s t i g a t i o n of a l i q u i d a l k a l i metal-halogen s o l u t i o n .

I1 .TFEORETICAL CONSIDEXATIONS The primary f o c u s of t h i s s t u d y i s t h e phenomenon of e l e c t r o n l o c a l i z a t i o n a s s o c i a t e d w i t h t h e M-NPl t r a n s i t i o n . S p e c i f i c a l l y , we address b i o questions.

F i r s t , &o t h e conduction e l e c t r o n s t a t e s become l o c a l i z e d f o r s u f f i c i e n t l y 1011

concentxationa of eexcss Cs 5 n CSI?

Second, i f l o c a l i z e d s t a t e s do form, what i s t h e i r microscopic s t r u c t u r e ? Because t h e NI% p r o p e r t i e s a r e s t r o n g l y i n f l u e n - ced by t h e microscopic e l e c t r o n i c s t r u c - t u r e and dynamics a c t i n g tlrrough t h e magnetic hyperfine i n t e r a c t i o n , N M R s t u d i e s can make a unique c o n t r i b u t i o n t o t h e r e s o l u t i o n of t h e s e questions.

!be h y p e r f i n e i n t e r a c t i o n w i t h t h e e l e c t r o n s introduced by excess C s i n C s I p r o v i d e s both a s h i f t of t h e NMR and n u c l e a r magnetic r e l a x a t i o n . For degene-

'permanent address : Fachhochschule, FB 06 P ~ Y s . Technik and Physik Dept. T . U . ~iinchen. West Germany

r a t e i t i n e r a n t e l e c t r o n s , t h e s h i f t i s t h e f a m i l i a r IZnicht s h i f t

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n .

I n t h e o p p s i t e l i m i t of d i l u t e l o c a l i z e d e l e c - t r o n s o i n s , t h e Curie-1a1.1 s u s c e p t i b i l i t y l e a d s t o a s h i f t

D H

(2)

where c i s t h e mole f r a c t i o n of excess C s , 1 i s t h e h y p e r f i n e i n t e r a c t i o n aver- aged over a l l r e s o n a n t n u c l e i , r e fn a r e t h e e l e c t r o n i c and n u c l e a r gyromag- n e t i c r a t i o s , r e s p e c t i v e l y , and S i s t h e e l e c t r o n spin. I n each c a s e t h e s h i f t can be seen t o be determined by t h e e f f e c t i v e number of unpaired s p i n s v i a t h e s u s c e p t i - b i l i t y and t h e i r s p a t i a l . d i s t r i b u t i o n v i a

< I ~ ( o > l

^{2>For X.}

S l e c t r o n l o c a l i z a t i o n i s probed most d i r e c t l y by measurements of t h e n u c l e a r s p i n - l a t t i c e r e l a x a t i o n r a t e T h i s i s because t h e r a t e depends on a c o r r e l - a t i o n time Z c h a r a c t e r i z i n g f l u c t u a t i o n s of t h e l o c a l h y p e r f i n e f i e l d . For degener- a t e elec-trons, Whe rate ettn be ~XtpresseB.

where (l/T1)Ko,r. i s t h e Korringa r a t e de- termined completely by t h e Knight s h i f t . and t h e temperature, and N(EF) i s t h e d e n s i t y of s t a t e s a t t h e Fermi l e v e l . The second f a c t o r i n Eq. ( 3 ) i s of o r d e r u n i t y i n nearly-free-electron.metals where t h e mean-free-path exceeds t h e average i n t e r - atomic s e p a r a t i o n , and r e l a x a t i o n r a t e s c l o s e t o (l/T1 )Korr.are commonly observed

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1980810

i n suck c a s e s . IIo::iever, t h e o n s e t of l o c a l - i z a t i o n l e n g t h e n s 'I: l e a d i n g t o enha2ce- n e n t r e l a t i v e t o t h e Icorringa r a t e / 5 / . For l o c a l i z e d e l e c t r o n s i n a l i q u i d t h e r a t e i s given by

2 S ( S + I ) ( A / ) ~ ) ~ Y

T,

=

S

(4

and i s t h e r e f o r e a l s o p r o g o r t i o n a l t o t h e hyperf i n e c o r r e l a t i o n t i n e ,

T- ILI. ZXPERITIEBTAL PtXSJLTS AFD DISCUSSIOA 3 3 ~ s and 1 2 7 ~ 1PER measurements were c a r r i e d o u t w i t h a c o h e r e n t p u l s e d FPlR s p e c t r o m e t e r o p e ~ a t i n g a t a f r e q u e n c y of 9.7 NIIz. The h i g h l y r e a c t i v e sara9les con- t a i n i n g e x c e s s C s were s e a l e d i n A l 2 O 3 o r Be0 ceramic c e l l s b r a z e d t o lJb c l o s u r e assemblies. S m p l e s of pure C s I o r C s I - I u e r e s e a l e d i n f u s e d q u a r t z ampoules. Re-

sonance s h i f t s measured r e l a t i v e t o aque- ous s o l u t i o n s of 1 3 3 ~ s ~ ~ and K ' * ~ I (cor- r e c t e d t o i n f i n i t e d i l u t i o n ) a r e shown i n

Pig. 1. ATOMIC % 1

MOLE X CS MOLE K I

P i g u r e 1, 1 3 3 ~ s ( 0 )and 1 2 7 ~ ( ~ ) r e s o n a n c e s h i f t s v e r s u s composition i n C s - I l i q u i d s o l u t i o n s a t 6 4 0 ' ~

The most remarkable f e a t u r e s of t h e s h i f t d a t a a r e t h e s t r o n g , n o n l i n e a r r e - d u c t i o n of t h e 1 3 3 ~ s s h i f t on adding I t o C s , and t h e r e l a t i v e l y l a r g e s h i f t ob- s e r v e d f o r 1 2 7 ~ i n Cs-rich s o l u t i o n s . The r e d u c t i o n of t h e C s Knight s h i f t a t low I

cl?ncentra-kions i s c h a r a c t e r i z e d by

r

^s

('~/c)AK/x @

-

0.65 which i s s i m i l a r t o t h e v a l u e = -0.8 r e n o r t e d f o r 0 i n Cs/6/.

The 1 2 7 ~ s h i f t i n Cs-rich s o l u t i o n s ( i , e . CsO. 6(CsI)0,4) i s comparable u i t h t h e Knight s h i f t s observed f o r many of t h e l i g h t e r m e t a l s . T h i s o b s e r v a t i o n shows t h a t t h e r e i s s u b s t a n t i a l p e n e t r a t i o n of conduction e l e c t r o n charge t o t h e n u c l e u s of t h e I- i o n i n s o l u t i o n . For low concen- t r a t i o n s of e x c e s s C s , b o t h s h i f t s de- c r e a s e n e a r l y l i n e a r l y t o t h e s m a l l chemi- c a l s h i f t s of s t o i c h i o m e t r i c l i q u i d G s I . The chemical s h i f t s a r e - 6 i 3 ? = 340 f 25 3pm and -CiIz7 = 435 5 20 ppm.

The 1 3 3 ~ s r e l a x a t i o n r a t e s shown i n P i g . 2 a r e weakly dependent on concentra- t i o n i n Cs-rich s o l u t i o n s b u t r i s e t o a s h a r p geak a t about 55% e x c e s s C s . I n t h e r e g i o n of t h e peak we have measured t h e i n v e r s e spin-phase memory time T2 which _*

-

-

can be expected t o e q u a l TI i n a homogene- ous l i q u i d . The peak 1 3 3 ~ s r e l a x a t i o n r a t e i s more t h a n 5 o r d e r s of magnitude g r e a t e r t h a n t h e v a l u e l/Tq =(0.140 5 0.005's-~

measured f o r p u r e l i q u i d C s I . The 127=

r a t e ?/TI shows a s i m i l a r maximum b u t t h e o v e r a l l magnitude i s much lower and t h e r e i s a s u b s t a n t i a l background r e l a x a t i o n r a t e , even i n p u r e C s I . The o v e r a l l lower r a t e shows, a s do t h e s h i f t v a l u e s , that t h e e x c e s s e l e c t r o n s a r e most s t r o n g l y a s s o c i a t e d w i t h 1 3 3 ~ s n u c l e i . The back- ground r e l a x a t i o n i s a t t r i b u t e d t o nu- c l e a r e l e c t r i c quadrupolar r e l a x a t i o n which, owing t o a l a r g e r quadrupole moment, i s much more e f f e c t i v e f o r t h a n f o r 133cs,

The 1 3 3 ~ s r a t e s f o r s o l u t i o n s w i t h ex- c e s s I a r e @8

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& d e r s of magnitude g r e a t e r t h a n observed f o r C s I , b u t t h e y remain f a r below t h e v a l u e s o b t a i n e a with comparable amounts of e x c e s s metal. We a t t r i b u t e t h i s r e - l a x a t i o n t o e l e c t r i c quadrupolar r e l a x - a t i o n a s s o c i a t e d w i t h t h e f o r m a t i o n of

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s c l u t i o n s a t 6 4 0 ~ ~ . "or ^";' -"CS, open c i r c l e s d e c o t e s p i n - l a t t i c e - e l a x a t i o n r a t e s ?/TI and c l o s e d c i ~ c l e s d z r o t e i z - v e r s e sniz.,-?har;e rnemory t i m e s 'I/$

e x c e s s I o m o l e s , n r e s u x a h l y b e c a u s e of e x t r e m e l y r a p i d q u a d r u ~ o l a r r e l - a x a t i o u r a t e s .

The ?en?-, i n t h e '"CS r a t e i s s t r i k i n g e v i d e n c e f o r e l e c t r o n i c ? . o c a l i z a t i o s on o r n e a r t h c ~ s * i o n s . S i n c e t h e s h i f t d-e- c r e a s e s monotonLcally i n t h e r e ~ i o n of t h e p e a k , t h e enhanced r e l a s a t i o r : can o n l y be due t o g r e a t l y i n c r e a s e d hyper- f i n e c o r r e l a t i o n t i m e s . Tne v m i a t i o r . of

a s s o c i a t e d w i t h t h e o3se?; of l o c a l - i z a t i o n can be deduced f r o n t h e o b s e r v e d r e l a x a t i o n r a t e s u s i n g Eq. (3). The K o r r i n g a r a t e (1 /!TI

IKorr.

i s c a l c u l a t e d from t h e K n i g h t s h i f t and B(EF) i s assm- e d t o vary with c o n c e n t r a t i o n a c c o r d i n g t o f r e e e l e c t r o n t h e o r y . OW deduced v a l u e s of Z were n o r m a l i z e d t o t h e

vzilue '7d = a/% f o r 3 u r e cz ~ " l h c r e a i s

A

t'ne neax i n t e r i o n i c s e - o a r a t i o n and vj, i s t h e F e r n i v e l o c i t y

/5/,

The r e s u l t s , nre- s e n t e d i n Fig. 5 show a g r a d u a l i n c r e a s e of Z o v e r more t h a n two d e c a d e s ' n e s r , ~ e e ~ Qwe C s and t h e r e g i o n of t h e r e l a x a t i o n r a t e uealr. -;Talues of % may a l s o be de- duced us in^ Eq. (u) f o r l o c a l i z e d s t a t e s , b u t it i s f i r s t n e c e s s a r y t o d i s c u s s he

s t r u c t u r e of t h e s e s t a t e s a s i n f e r r e d f r o n t h e h y p e r f i n e cous1il;g c o n s t a c i s .

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L"i[;ure 3. i$me:Si-:e f i e l d c o r r e l a t i o n -cines f o r '"CE i n l i q u i d C s - C s I s o l u - t i o n s a t 61&O0C. Ogen p o i n t s d e n o t e v a l u e s derived. from Xq. ( 3 ) b a s e d on enhanced i t i n e r a c t n o d e l . C l o s e d p o i n t s d e n o t e v a l u e s d e r i v e d from l o c a l i z e d model X q . (4) assuming Y-center a n a l o g u e s .

T h e i n i t i a l r a t e of i n c r e a s e of t h e 1-33~s r e s o n a n c e s h i f t a s C s i s added t o C s I y i e l d s , from Eq. (2), BCs =

(3.32 f 0.33) I O - ~ ~ J , o r a b o u t 20~6 of t h e a t o m i c 6s h y p e r f i n e c o u p l i n g

The i n i t i a l s l o p e of t h e 1 2 7 ~ s h i f t c o r r e s p o n d s t o

AIDCs

⁼ 0.33

- +

0.05.

These v a l u e s a p p a r e n t l y p r e c l u d e i d e n t i - f i c a t i o n of t h e l o c a l i z e d s t a t e s as neu- t r a l atoms s i n c e t h e r e i s i n s u f f i c i e n t c h a r g e d e n s i t y a t e i t h e r t h e C s o r t h e

I n u c l e i . I n c o n t r a s t , r e a s o n a b l e c h a r g e d e ~ s i t i e s can be i n f e r r e d by follov!ing t h e s u g g e s t i o n of P i t z e r /7/ t h a t F - c e n t e r - l i k e s t a t e s a s e formed. F o r an e l e c t r o n i n an a n i o n v a c a n c y s u r r o u n d e d by z

( e 5) CS+ i o n s , a ~ r o b a ' o i l i t y a r n ~ l i t u d e

I y ( c s ) l 2 *

^0.03

1

~ ( c s ) ! Zatoa i s o b t a i n e d . F u r t h e r , f o ? r o u g h l y 1 2 second n e i p h b o r

1- i o n s ,

I Y(1)I

^{e o.lq}

¹ Y ( c ~ ) 1

^2. m ~ e s e v a l u e s z r e c o n s i s t e n t ;?it5 t h e f i r s t -

and. s e c o n d - n e i s h b o r h y p e r f i n e f i e l d s measured f o r F - c e n t e r s i n a l k a l i h a l i d e c r y s t a l s / 8 / .

riow t h e m a n - s q u a r e h m e r f i a e c o u p l i n g is r e l a t e d t o t b e mean v a l u e b y 7z - l x 2 , ~ a d f o r P - c e a t e r analoguer; v i t h ^I, 6 i J e f i n d ( A h ) 2 1.65 x

l o q 8

s-I. Using t h i s v a l u e i n t h e e x p r e s s i o n q i v e n i n l q . (4) f o r r e l a x a t i o n i n t h e l o c a l i z e d l i n i t , T:e o b t a i n t h e '% - v a l u e s shown i n P i g . 3 . It i s g r a t i f y i n g t h a t t h e s e v a l u e s a c m e w i t h t h o s e deduced from t h e K o r r i n g a en- hancement i n t h e r e g i o n of t h e r e l a x a t i o n peak. The c o r r e l a t i o n t i m e s c o n t i n u e t o d e c r e a s e w i t h C s c o n c e n t r a t i o n down t o t h e l o ~ e s ' i concen"irations s ' i u d i e s (shout

c.5.75

e x c e s s C s ) , t h u s i n d i c a t i n g t h e p r e - s e n c e of c o u p l i n g between l o c a l i z e d e l e c - t r o n s a t d i s t a n c e s uc t o 20-30

8.

I n s m a r y , we have found t h a t t h e M-NIT t r a n s i t i o n i n l i q u i d C s - C s I s o l u - t i o n s i s accompanied b y p r o g r e s s i v e l y s t r o n g e r e l e c t r o n i c l o c a l i z a t i o n . The hy-perfine c o u p l i n g s and dynamical p r o p e r - t i e s of t h e s e s t a t e s s u g g e s t a s t r u c t u r a l model of weakly i n t e r a c t i n g F - c e n t e r a n a l o g u e s a t lord c o n c e n t r a t i o n s of e x c e s s m e t a l . T h i s r e s u l t i m p l i e s t h a t l o c a l i z a - t i o n i n t h i s system i s d r i v e n b y t h e s t r o n g coulomb i n t e r a c t i o n s w i t h t h e l o c a l c o n f i g u a t i o n of i o n s r a t h e r t h a n b y t h e a b s e n c e of long-range o r d e r .

F o r a re vie:.^ o f d a t a un t o 1964 s e e Zredi-;, i n K o l t e n S a l t Chemistry, ed. F!. Y l s n d c s ( I c t e - s c i e n c e , >Te~-j York, 1961: )

.

F. IIensel: Adv. P l i ~ c i c s

28,

555 (1979).

!!.

:>!. r J a r r e n , Jr.

,

i n Advances

a

N a l t e r , _ S 3 C h e m i s t r y , cd. C-. Nzmantov and J. B r a u x s t e i n (Plenum P r e s s , Bev~

York, 1 9 8 0 ) , Vol. 4..

!?osztoczy and

n.

C u b i c c i o t t i , 2. Phys. Chen.

2,

1 6 (1965) 1l.V. !Lerrec, Jr.

,

Phys. Rev. B

2,

3708 (1971)-

I.?. Tiost, 3.F.bi. S e m o u r , and G.I.

S t y l e s , J. 1Ju-cL. Plat.

3,

55

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K.S. P i t z e r , J. Am. Chem. SOC. 8 4 , 2025 (1962).

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